THE MIDCONTINENT RIFT
SYSTEM IN IOWA
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LOWER RED CLASTICS GROUP |
Introduction to Lower Red Clastics Group
The rocks encountered at depth between 3153 and 5310 m (10,510 to 17,700 ft) in the Eischeid well were informally named the Lower Red Clastic Group. The group was divided into three component units by Witzke (1990), from the top, Unit D, Unit C, and Unit B. To view a discussion of each of the induvidual units that comprise the Upper Red Clastics Group, click on the appropriate name on the following chart.
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LOWER RED CLASTICS GROUP STRATIGRAPHY
MIDCONTINENT RIFT SYSTEM ROCKS OF IOWA
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(click for map of thickness of Lower Red Clastics in flanking basins)
Discussion of the Lower Red Clastic Group.
The Lower Red Clastic Group closely resembles and may correlate with the Oronto Group in the Lake Superior area. The biggest differences between the two groups are the coarser-grained facies preserved in the Copper Harbor Conglomerate, the basal unit in the Oronto Group, and the dominance of quartz sand in Unit B, the basal unit of the Lower Red Clastic Group. The Copper Harbor is dominated by a conglomeratic facies composed primarily of mafic and felsic volcanic rock clasts. Associated sandstone facies also are dominated by volcanic rock fragments. Unit B is dominated by a sandstone facies that displays the highest quartzose grain content (87%) of any Red Clastic unit in the Eischeid well (click here for QFL information). This disparity can be explained by the positions of the two units relative to the central horst of the Midcontinent Rift. The Copper Harbor Conglomerate exposures are located on the central horst, which was an axial graben at the time of Copper Harbor deposition. The volcanic-dominated rocks of the Copper Harbor Conglomerate were deposited by alluvial fans that were derived in large part from the erosion of Keweenawan volcanic rocks capping the footwalls of the graben-bounding normal faults. In contrast, the Eischeid well, and Unit B, are located in one of the clastic basins that flank the central horst, and Unit B was therefore deposited outside of the central graben. Unit B is dominated by quartzose sediment derived from granitic rocks, the most common basement rock lithology in the MRS region, and was deposited by rivers that flowed towards the axis of the rift. As these rivers approached the axial graben they may have cut through footwall-capping volcanics, depositing Copper Harbor lithologies in the graben.
Unit C is lithologically and sedimentologically very similar to the Nonesuch Formation of the Oronto Group. The fine grained components of Unit C display unidirectional, bi-directional, symmetrical, and trough cross stratification and small scale amalgamated hummocky cross stratification. Shrinkage cracks are also present in this interval (Ludvigson and others, 1990). Milavec (1986) identified similar features in the Nonesuch Formation.
The Nonesuch Formation has been interpreted as a lake deposit by most workers (e.g., Elmore, 1981; Daniels, 1982; Milavec, 1986). However, some workers have suggested that it may have been deposited in a marine environment (Burnieand others, 1972; Hieshima and others, 1989; and Hieshima and Pratt, 1991). In either case, the Nonesuch appears to have been deposited in a standing body of water. Structures observed in cores of Unit C also suggest deposition in a standing body of water (Ludvigson and others, 1990). The presence of these lake deposits in the Eischeid well indicate that the body of water was not confined to the central horst, the position where correlative Nonesuch deposits are observed. Interpretation of petroleum industry seismic data suggests that Unit C may extend at least 11 km (7 mi) west of the Iowa Horst in the Defiance Basin, and perhaps as much as 24 km (15 mi) or more.
Unit C cores show significant deformation that is not present in cores of other intervals. The entire interval of Core #4 (4813 to 4817 m; 16,043 to 16,058 ft) displays a high angle of tectonic dip, ranging from 65 degrees to vertical or slightly overturned (Ludvigson and others, 1990). Additionally, Ludvigson and Spry (1990) reported a silckensided brittle fault surface in this interval, with a set of rough facets resulting from secondary mineral growth during faulting, indicative of reverse faulting. They also identified smaller pull-apart microfaults/veinlets displaying a reverse sense of displacement and folds with white calcite veins on the outer circumference of the fold hinge which they interpreted as filled extension fractures.
The black coloration in Unit C, like the Nonesuch Formation, is primarily the product of disseminated organic carbon, along with minor pyrite. Palacas and others (1990) reported total organic carbon (TOC) values as high as 1.4% (averaging 0.6%) in the dark shales and siltstones from cores of Unit C. The Nonesuch Formation has yielded maximum TOC values of almost 4% (Hieshima et al., 1989). The petroleum source rock potential of Unit C is discussed later.
Unit D occupies the same stratigraphic position in the Eischeid well as the Freda Formation in the Oronto Group of the Lake Superior area. Interpretation of Core #1 from Unit D suggested (Ludvigson and others 1990) that the unit was deposited in a fluvial environment. A similar interpretation has been proposed for the Freda Formation by many workers (e.g., Daniels, 1982; Morey and Ojakangas, 1982b). Petrographic studies of Unit C revealed a mean detrital composition of Q-76 F-21 L-3 compared to a mean composition of Q-52 F-18 L-30 for seven Freda exposures reported by Daniels (1982). The lithic component of the Freda is dominated (72%) by volcanic rock fragments. The higher volcanic lithic component indicates that volcanic rocks were exposed in the Freda source area, and similar lithologies were not exposed in the source area of Unit D.
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